Archaean continents derived from Hadean oceanic crust

As DNA is to tracing human evolution and migration, so various isotope systems are to the evolution of the Earth. One of the most fruitful is the samarium-neodymium (Sm-Nd) system. The decay of 147Sm to 143Nd is used in dating rocks across the full range of Earth history, given coeval rocks with a suitable range of Sm/Nd ratios, because the decay has a long half life (1.06 x 1011 years). However, samarium has another radioactive isotope 147Sm with a half life that is a thousand times shorter (1.06 x 108 years). So it remains only as a minute proportion of the total Sm in rocks, most having decayed since it was formed in a pre-Solar System supernova. But its daughter isotope 142Nd is present in easily measurable quantities, having accumulated from 147Sm decay over the first few hundred million years of Earth’s history; i.e. during the Hadean and earliest Archaean Eons. It is this fact that allows geochemists to get an indirect ‘handle’ on events that took place in the Earth’s earliest, largely vanished history. The principle behind this approach is that when an ancient rock undergoes partial melting to produce a younger magma the rock that crystallizes from it inherits the relative proportions of Nd isotopes of its source and thereby carries a record of the earlier history.

The eastern shore of Hudson Bay in Canada hosts the oldest tangible geology known, in form of some metamorphosed basaltic rocks dated at 4200 Ma old known as the Nuvvuagittuq Greenstone Belt – the only known Hadean rocks. They occur in a tiny (20 km2) patch associated with gneisses of tonalite-trondjhemits-granodiorite composition that are dated between 3760 and 3350 Ma. Engulfing both are younger (2800 to 2500 Ma) Archaean plutonic igneous rocks of felsic composition. Jonathan O’Neil and Richard Carlson of the University of Ottawa, Canada and the Carnegie Institution for Science, Washington DC, USA respectively, measured proportions of Nd isotopes in both sets of felsic igneous rocks (O’Neil, J. & Carlson, R.W. 2017. Building Archean cratons from Hadean mafic crust. Science, v. 355, p. 1199-1202; doi:10.1126/science.aah3823).

The oldest gneisses contained relative proportions of 142Nd commensurate with them having been formed by partial melting of the Hadean mafic rocks about a few hundred million years after they had been erupted to form the oldest known crust; no surprise there. However, the dominant components of the local continental crust that are about a billion years younger also contain about the same relative proportions of 142Nd. A reasonable conclusion is that the Archaean continental crust of NE Canada formed by repeated melting of mafic crust of Hadean age over a period of 1.5 billion years. The modern Earth continually replenishes its oceanic crust over about 200 Ma due to plate tectonics. During the Archaean mantle dynamics would have been driven faster by much higher internal heat production. Had this involved simply faster plate tectonics the outermost skin of mafic crust would have been resorbed into the mantle even faster. By the end of the Archaean (2500 Ma) barely any Hadean crust should have been available to produce felsic magmas. But clearly at least some did linger, adding more weight to the idea that plate tectonics did not operate during the Hadean and Archaean Eons. See Formation of continents without subduction below.

Hi Steve. The point I was trying to make, and I think that Hamilton was making, is that the melabasaltic protocrust was not created by actualistic plate tectonic process, i.e. seafloor spreading, but was formed globally on top of the Hadean “magma sea” more or less synchronously and that the felsic Archean basal crust (as well as the mafic and ultramafic greenstone belts), were extracted from that protocrust by partial melting. Hamilton further argued that the various (e.g. Nd-Sm) model ages of Neoarchean and Paleoproterozoic “juvenile crust” were not mantle extraction ages but actually record the ages that the “juvenile” crust was extracted from the protocrust; in other words it was not really juvenile in the sense of being derived directly from the mantle, but was evolved and derived from the mafic protocrust (cpx-garnet-hbl granulite). Plagioclase and olivine cannot co-exist deeper than about 30km, which means that plag can’t come out of the mantle.

In the past, geophysics ruled (and overruled the geologists). Nowadays the geochemists rule, with all of their trace element plots that only they can understand/interpret, and the geologists again are ignored.

Have you read this geochem paper, which indicates that there is no essential difference between MORB (final melt at ~zero depth) and OIB (final melt depending on the thickness of the overlying oceanic lithosphere)?

Good of you to add a further comment, Bill. I hope it draws others in – both of yours deserve wider discussion.

One of the best sources outlining the way views on Palaeoarchaean and Hadean tectonics are changing that I have come across is a recent issue of the free-access journal Geoscience Frontiers, part of which was devoted to ‘lid tectonics’. I sketched out some of the important (to me) points in https://earth-pages.co.uk/2017/12/21/lid-tectonics-on-earth/ , which I think may encompass some of the points you and Hamilton have made.

Having given up on Achaean matters, both petrogenetic and tectonic, in the mid-1980s – it had then become a stagnant topic, in my view – I only kept up with developments fitfully. The Geoscience Frontiers papers really spice it up, but I have lost the geochemical wherewithal to enter a proper dialogue, I’m afraid. Advanced years also play a part!

Thanks also for the link to Yaoling Niu et al, which is in the ‘lid tectonics’ genre. It was good to see Mike O’Hara back in the saddle, and also Marge Wilson. O’Hara had a giant reputation when he was at Edinburgh doing experimental petrology, and he must be pretty ancient – Hope for us all!

The title is misleading. Mafic crust is not necessarily oceanic crust.

That the Eoarchean earth was covered by a “thick global melabasaltic protocrust”, and that the Archean TTGs belts were derived from it, is precisely the argument that Warren Hamilton has been making for the past twenty years or so.

“The magmatic and tectonic processes of the pre–2.5 Ga hot, young Earth differed profoundly from those of the modern planet. The ancient rocks differ strikingly in individual and collective composition, occurrence, association, and structure from modern rocks. Widespread forcing of Archean geology into plate-tectonic frameworks reflects unwarranted faith in uniformitarianism and in inappropriate chemical discriminants, and disregard for the lack of features that characterize plate interactions. Archean crust records extreme and prolonged internal mobility and was far too weak and mobile to behave as rigid plates, required, by definition, for plate tectonics. None of the geologic indicators of subduction, arc magmatism, and continental sundering, separation, and convergence have been documented. No Archean oceanic crust or mantle has been recognized, and the only known basement to supracrustal rocks, including the thick basalts, high-Mg basalts, and ultramafic lavas that typify greenstone successions, consists of tonalite-trondhjemite-granodiorite (TTG) migmatites and gneisses. A thick global melabasaltic protocrust likely formed by ca. 4.45 Ga, and from it TTG suites were extracted by partial melting over the next 2 b.y.”

Yes, Bill, you have a point, and I too admire Warren Hamilton’s pithy contributions. However, it is a moot point, as the weakness of Eoarchaen lithosphere may well have led to global topography that was so muted that it was widely flooded – perhaps a ‘waterworld’!
regards
Steve Drury

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